The use of catalytic processes in chemical reactions serves several purposes. Among these purposes are the lowering of barriers to a particular reaction (the "activation energy") so that the reaction can proceed under relatively mild conditions, and the shifting of dynamic equilibrium such as to increase the yield of a specific set of products which are kinetically disfavored under noncatalytic conditions. Once a catalytic system is implemented, it may be further fine-tuned by the use of promoters and/or inhibitors, such as those described in Norskov et al., Surf. Sci. 137: 65 (1984). It is highly desirable to be able to use these adspecies to promote the reactions leading to certain products, yet poison undesirable pathways at the same time.
One particularly important catalytic process is known as the Fischer-Tropsch reaction comprises the catalytic production of hydrocarbons and other oxygenated compounds from synthetic gases, specifically carbon monoxide (CO) and hydrogen gas (H.sub.2). By this reaction, the efficient mass production of fuel is possible, and this fuel could provide a much needed alternative to the use of petroleum products. The importance of having a safe and productive domestic method of converting any combustible carbon-containing source into usable energy cannot be understated.
The major drawback in the mass production of fuel through the Fischer-Tropsch (or F-T) synthesis thus far has been the high statistical distribution of products which result from the reaction. In broadest terms, this reaction can be described as follows: EQU CO+H.sub.2 .fwdarw.hydrocarbons+CO.sub.2 +H.sub.2 O (I)
The F-T catalysts generally are chosen from the "d-block" of the periodic table, commonly known as the transition metals. The most commonly used of these metals for the reaction are iron, cobalt, nickel and ruthenium (as further described in Anderson, The Fischer-Tropsch Synthesis, Academic Press, Orlando, Fla., 1984).
The metal catalysts are employed in order to adsorb and then dissociate the H.sub.2 and CO gases in the reactions indicated as follows: EQU H.sub.2 +M.revreaction.H--M--H (II) EQU CO+M.revreaction.M--CO (III) EQU M--CO+M.revreaction.M--C+M--O (IV)
wherein M is the metal catalyst. Among the many possible pathways in the overall F-T reaction, the individual dissociated atoms can react with each other or with other compounds, leading to many possible products, just a few of which are indicated in the following reactions: EQU M--C+M--H.revreaction.M--CH+M (V) EQU M--CH+M--H.revreaction.M--CH.sub.2 +M (VI) EQU M--O+M--H.revreaction.M--OH+M (VII) EQU M--CO+M--H.revreaction.M--CHO+M (VIII) EQU M--CHO+M--H.revreaction.M--CHOH+M (IX)
These and other reactive pathways are desired in Rofer-DePoorter, Chem. Rev. 81: 447 (1981). Obviously, there are a larger number of products which can be produced in a Fischer-Tropsch synthesis.
The currently used catalytic systems allow for high selectivity only with regard to methane and methanol synthesis. If higher hydrocarbons are desired, one must be prepared to accept a distribution of products which includes both lighter and heavier hydrocarbon fractions along with the more desirable products. Oxygenated products (alcohols, ethers), branching, and degree of saturation all must be considered in determining the relative success of producing desired hydrocarbons from the synthetic gas.
Although major research efforts have been underway with the aim of narrowing the distribution of products, these efforts have had mixed levels of success. Included in these efforts are variation of supports, various poison/promoter combinations, and the use of molecular sieve-type zeolites. The zeolites generally are porous substrates in which the pores are windows which open into cavities or cages in the unit cell wherein the reaction takes place. These cages can thus hold the precursor syngas components and intermediates in configurations which can greatly narrow the product distribution and increase the potential for producing desired products. What has not yet been accomplished is the discovery of a way on a two-dimensional surface catalyst to configure the precursor species prior to the F-T reaction in order to enhance a narrow product distribution and ultimately provide for greater production of the desired hydrocarbons.